Americans are more likely than ever to reach age 90, redefining in a way what it means to be old.

People who are 90 or older have nearly tripled in number since 1980, to 1.9 million, according to last week’s first-ever census numbers on the age group. The trend is posing unique health challenges and adding to rising government costs for the strained Medicare and Social Security programs.

Joined by graying baby boomers, the oldest old are projected to increase to 8.7 million by midcentury — or one in 10 older Americans. That’s a big change from over a century ago, when fewer than 100,000 people reached 90.

Analysts attribute the increases in the 90-plus age group mostly to better nutrition and advances in medical care that have reduced heart disease and stroke. Still, the longer life spans present a fresh set of challenges for disabilities and chronic conditions such as arthritis, diabetes and Alzheimer’s disease.

Figures show that smaller states had the highest shares of their older Americans who were at least 90. North Dakota led the list, with about 7 percent of its 65-plus population over 90. It was followed by Connecticut, Iowa and South Dakota. In absolute numbers, California, Florida and Texas led the nation in the 90-plus population, each with more than 130,000.